Humidity tolerant scan lens

ABSTRACT

A lens has a body ( 1 ), reinforcing extensions ( 3   a   , 3   b ) and a clear aperture ( 9 ) surrounded on top and bottom by lens body. The lens is suitable for spot scanning. The lens is made of a water absorbing material, such as most polymers, particularly acrylate polymers. Aluminum sheets as vapor barriers are attached on each side of the lens body. This results in excellent resistance to change or distortion in high humidity environments.

TECHNICAL FIELD

This invention relates to lenses of configuration for light scanningthrough the lens, such as f-theta lens, the lenses being of materialwhich changes or distorts the optical function with increases inhumidity. This invention renders such lenses resistant to such changesor distortions.

BACKGROUND OF THE INVENTION

Materials forming lenses that change or distort with increases ofhumidity are typically plastics, especially acrylic plastics. Somehydrocarbon plastics resist water intake and are used in specialsituations for lenses, but the material are expensive and are difficultto mold to form the lens.

The prior art has addressed this problem in U.S. Pat. No. 5,409,360 toNakanishi, but the solution presented in that patent is one whichsignificantly restricts the characteristics and form of the lens sinceit is based on the premise that a lens fully permeated with water doesnot exhibit major distortion. Accordingly, the better lenses inaccordance with this patent would be tall and thin.

Lenses of this invention may differ little from ordinary form for scanlenses, but have vapor barriers applied to render the lens highlyresistant to increases in humidity.

DISCLOSURE OF THE INVENTION

A lens of this invention has a long dimension, an optical transmissiondimension perpendicular to the long dimension, and a height dimensionperpendicular to both the long dimension and the optical transmissiondimension. The lens has a clear aperture along the long dimension of atlease 35 millimeters (mm), the clear aperture having a height that isless than 32 mm. The clear aperture is surrounded top and bottom by lensbody having height on each side of at least 1.5 mm.

On the top and bottom of this lens is a layer of a vapor barrier, whichmay be a sheet of metal or other imperious material attached byadhesive. Such lenses are very stable optically with changes in humidityup to almost saturation humidity. Where a lens is thin in the lighttransmitting direction, the barrier might be omitted at such thinpoints, as the significant optical distortion does not occur where thelens is thin enough to become fully saturated.

BRIEF DESCRIPTION OF THE DRAWINGS

The details of this invention will be described in connection with theaccompanying drawings, in which

FIG. 1 is a perspective view showing the lens with vapor barrier;

FIG. 2 is a is a second perspective view from a higher angle than FIG. 1showing vapor barrier sheet spaced away for purposes of illustration,

FIG. 3. a top view looking down on the light transmission plane showinga cross section of the lens through the clear aperture;

FIG. 4 is a side view from the optical transmission path beforeapplication of a vapor barrier;

FIG. 5 is a bottom view looking up on the light transmission planebefore application of a vapor barrier;

FIG. 6 is a front view from the optical transmission path showing indotted outline the clear aperture region before application of a vaporbarrier;

FIG. 7 is a side view of FIG. 5 from the right side,

FIG. 8 is a sectioned view through the center of the lens, also of FIG.5 from the right;

FIG. 9 is a plot of data from a representative application without thevapor barrier of this invention; and

FIG. 10 is a plot of data from the same representation application asthat of the FIG. 9 data with the vapor barrier of this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The lens of this invention is shown illustratively in FIG. 1. It ischaracterized by being relatively long and narrow. It has an opticalbody 1 and planar extensions 3 a (FIG. 2) and 3 b, which provide addedrigidity. Extensions 3 a and 3 b are molded integrally with optical body1 for molding efficiency and for immediate, strong contact with body 1.The lens has a top barrier layer 5 a and a bottom barrier layer 5 b,attached by adhesive in this embodiment.

As illustrated in FIG. 2, barrier layers 5 a and 5 b are substantiallycoextensive with the surface of extensions 3 a and 3 b respectively, andextensions 3 a and 3 b have portions that cover optical body 1.Accordingly, the top and bottom surfaces in the height dimension ofoptical body 1 are covered by barrier layers 5 a and 5 b in the completelens, such as shown in FIG. 1.

FIG. 3 illustrates, by cross section through the lens, the optical body1 and the lower planar extension 3 b. Although this invention isunderstood to be generally applicable to the type of lenses used forscanning, for purposes of this illustrative embodiment, the outersurface 1 a of body 1 has a circular radius of 235 mm and the innersurface 1 b of body 1 has a circular radius of 382 mm. With respect toFIG. 3 the length dimension of the lens is the horizontal dimension (thex direction in the insert), and the light transmission dimension is thevertical dimension (the y direction in the insert). The linear,horizontal dimension of the curved lens part is 152 mm.

Non-curved, rectangular end parts 1 c and 1 cc arc for positionreferencing and other support of the lens.

The major dimensions of FIG. 3 are generally to scale. It is apparentthat the optical body 1 is relatively thick throughout its length,although the thickness increases significantly between the middle andthinner ends. Such a lens has positive optical power. A diverging ornegative lens would be thinner in the middle than at the ends. Thisinvention applies to all scan lenses in general.

FIG. 4 is a back, side view with respect to FIG. 3 but not sectioned andwithout the barrier layers. Supporting parts 1 c and 1 cc arerectangular and extend across the lens. Part of the outer surface ofparts 1 c and 1 cc may be a reference or critical dimension from whichother part of the lens are to be located. Such referencing is standardand therefore will not be further discussed.

Element 7 is a gate for injection molding. It may be removed from thefinal lens if desired.

The insert in FIG. 4 is to define the long dimension of the lens as xand the height dimension of the lens as z. The major dimensions of FIGS.3 though 5 are generally to scale.

FIG. 5 is a bottom view with respect to FIG. 3. It illustrates bottomextension 3 b as having a center, rectangular small locating extension 3bb for locating the lens during use. Extension 3 bb is not shown inother views for purposes of clarity and because alternative elements tolocate the lens are entirely consistent with this invention. Similarly,FIG. 5 shows small bumps 1 d and 1 dd on supporting parts 1 c and 1 ccrespectively. These are alternative locating elements. Such locatingelements are standard and therefore will not be further discussed.

FIG. 6 is a view toward the lens as light exits the lens. It is a frontside view with respect to FIG. 3, but not sectioned and without thebarrier layers. Two, spaced positioning bumps 1 d and 1 dd are onopposite ends.

Shown in dotted outline in FIG. 6 is the optical aperture 9, which isthe part of the lens used for accurate optical scanning. In ordinaryuse, light might only be directed to the optical aperture 9, since thatis the part of the lens to be used for focusing. In this embodiment, theoptical aperture is generally centered in the lens, both laterally andin height, is 145 mm in the long direction and 5 mm in the heightdirection. The entire height of the lens, exclusive of the vaporbarriers, is 14 mm.

Accordingly, in this embodiment the linear length of the opticalaperture is almost 30 times the height of the optical aperture.Similarly, in this embodiment the height of the lens body is almostthree times the height of the optical aperture. However, it isunderstood that lens body as little as 1.5 mm in the height dimension oneach side of the optical aperture is effective, although preferably thatdimension should be at least 2.5 mm on each side. Material near theoutside of the mold exhibits optical distortion and is thereforeunsuitable for use as part of the optical aperture.

FIG. 7 is a side view of FIG. 5 and FIG. 8 is also a side view withrespect to FIG. 5, but sectioned in the center of the lens. They aregenerally to scale and therefore illustrate the overall dimensions.Surface 1 a is shown as concave. Specifically, it may be a true toric.In other lenses it might be convex, and the variations in curvaturealong the length define the focusing of the lens. Similarly, surface 1 bis shown concave and is typically toric or quasi toric. Althoughdefining the surfaces of a lens is specialized and complex, with respectto this invention it may be standard and will not be further discussed.

Barrier layers 5 a and 5 b in this embodiment are 0.178 mm thickaluminum foil with acrylic adhesive on the surface which contacts thesurfaces 3 a and 3 b respectively. The remainder of the lens asdescribed, which are lens body 1, extensions 3 a and 3 b, end parts 1 cand 1 cc, location extension 3 bb, and bumps 3 d and 3 dd are a singleintegral member formed of polyrnethyl methacrylate by simple injectionmolding or by injection molding with pressure assist, as is standard inthe molding art. Barrier layers 5 a and 5 b are applied by simplybringing the adhesive side of the barrier into direct contact with thesurfaces 3 a and 3 b respectively.

A typical use for the lens of this invention is part of a light spotscanning system on a photoconductive surface, as illustrated in FIG. 10of the foregoing U.S. Pat. No. 5,408,360. A small spot of light is movedacross the clear aperture to apply a small spot of light sequentiallyacross the photoconductive surface.

Lenses of this invention may be excellent f-theta lenses. Such lensesare well known in the art. They are used to direct a spot of lightacross a light sensitive surface with the light moving linearly with theangle (hence the reference to theta, a symbol commonly used for angle).Without the adjustment by the focusing of an f-theta lens, the linewould move in accordance with the cosine of the angle.

Since the internal sides of surfaces 3 a and 3 b not over body 1 are notcovered with a vapor barrier, the essential advantage is understood toresult from the top and bottom of body 1 having a vapor barrier, coupledwith the optical aperture being within body 1 and surrounded in theheight dimension on both top and bottom by body having height of atleast 1.5 mm. Similarly, where body 1 is thin in the opticaltransmission direction in certain lenses, the barrier layer may not benecessary over such locations as significant optical distortion is notnormally realized where a lens becomes saturated across it full width inthe optical transmission direction.

Alternative barrier layers can include any treatment or application,which is or becomes impervious to water. This would include paints andpossibly surface treatments by oxidation.

With respect to FIG. 9 and FIG. 10 data was obtained from an opticalsystem employing this invention originally at ambient temperature andhumidity and then subjected to 60 degrees C. and 90 percent relativehumidity. The spot size was measured both at the location representativeof the photoconductive drum where the spot was focused at the ambientconditions and at 1 mm equally spaced locations both plus and minus fromthat location. At ambient conditions the spot size measured becomeslarger as the measurement point departs from the location representativeof drum location.

At high humidity conditions the spot size at the photoconductor (drumplane 0) increased due to focal shift about 70 percent in size (about 70μm to about 120 μm). This data resulted in curves shown in FIG. 9. Asshown in FIG. 9, the day 13 and day 15 results of the 15 day test showand improvement, believed to be the result of the lens approachingsaturation. Where a vapor barrier was used, the results varied verylittle over a similar 15-day test, as shown in FIG. 10, thus clearlyshowing the value of this invention. (the curves are so close togetherthat they could not be meaningfully shown with their identifying symbol.The curves are very close together at the 0 drum plane.)

Accordingly, this invention is believed to have a wide applicability andvariability in form, particularly with respect to how the vapor barriersare formed.

What is claimed is:
 1. A lens of water absorbent material having alinear long dimension, an optical transmission dimension perpendicularto said linear long dimension and a height dimension perpendicular toboth said linear long dimension and said optical transmission dimension,said lens comprising a lens body containing a clear aperture, said lensbody terminating in the height dimension on opposite sides of said clearaperture, said clear aperture extending along said long dimension anlength of at least 35 mm and having a height in the height dimension ofless than 32 mm, said clear aperture being surrounded in the heightdimension by lens body of at least 1.5 mm on each side of said clearaperture, said lens body having vapor barriers located where said lensbody terminates on opposite sides in the height dimension of said clearaperture.
 2. The lens as in claim 1 in which said vapor barriers aresheets of water impervious material attached by adhesive.
 3. The lens asin claim 1 in which said lens has planar elements defining surfaceswhere said lens body terminates which increase rigidity of said lens,said vapor barriers being located on said surfaces.
 4. The lens as inclaim 2 in which said lens has planar elements defining surfaces wheresaid lens body terminates which increase rigidity of said lens, saidvapor barriers being located on said surfaces.
 5. The lens as in claim 1in which said lens body height on each side of said aperture is at least2.5 mm.
 6. The lens as in claim 2 in which said lens body height on eachside of said aperture is at least 2.5 mm.
 7. The lens as in claim 3 inwhich said lens body height on each side of said aperture is at least2.5 mm.
 8. The lens as in claim 4 in which said lens body height on eachside of said aperture is at least 2.5 mm.